Soil compaction increases the load-bearing capacity of the soil, adding stability. It also prevents soil settlement and water seepage, which can cause unnecessary maintenance costs and structure failure.
Before Compacting
Know your soil: Identify the soil group you are working with before you begin compacting, as different soil types have different maximum densities and optimum moisture levels. The three basic soil groups are: cohesive, granular and organic. Cohesive soils, such as clay, have particles that stick together. Granualar soils, such as sand, have no clay content, and crumble easily. Organic soils are not suitable for compaction.
Moisture: Before compacting, you need to determine the moisture content of the soil. Too little moisture results in inadequate compaction and too much moisture weakens stability. The easiest way to test the moisture content of soil is the “Hand Test.” Pick up a handful of soil, squeeze it, and then open your hand. You want the soil to be moldable and break into a few pieces when dropped. If the soil is powdery and shatters when dropped, it is too dry. If the soil leaves moisture in your hand and remains in one piece when dropped, it has too much moisture.
Applications
The desired level of compaction is best achieved by matching the soil type with its proper compaction method. Other factors must be considered as well, such as compaction specs and job site conditions.
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• Cohesive soils – clay is cohesive, its particles stick together. Therefore, a machine with a high impact force is required to ram the soil and force the air out, re-arranging the particles – a rammer is the best choice. For higher production work a roller or a reversible plate compactor can be used. • Granular soils – since granular soils are not cohesive and the particles require a shaking or vibratory action to move them, vibratory plates or rollers are the best choice.
Lift height and Machine Performance
Lift height (depth of the soil layer) is an important factor that affects machine performance and compaction cost. Vibratory and rammer-type equipment compact soil in the same direction: from top to bottom and bottom to top. As the machine hits the soil, the impact travels to the hard surface below and then returns upward. This sets all particles in motion and compaction takes place.
As the soil becomes compacted, the impact has a shorter distance to travel. More force returns to the machine, making it lift off the ground higher in its stroke cycle. If the lift is too deep, the machine will take longer to compact the soil and a layer within the lift will not be compacted.
Soil can also be over-compacted if the compactor makes too many passes (a pass is the machine going across a lift in one direction). Over-compaction is like constantly hitting concrete with a sledgehammer. Cracks will eventually appear, reducing density. This is a waste of man-hours and adds unnecessary wear to the machine.
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